Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Dark matter and energy explained by negative mass

  1. Dec 5, 2018 #1
  2. jcsd
  3. Dec 5, 2018 #2
    It looks like it just came out. I am still reading through it, but I don't see any discussion of what this negative mass dark fluid could be made of.

    Also, what would the geodesics around such a fluid look like?
    Last edited: Dec 5, 2018
  4. Dec 5, 2018 #3

    Very interesting paper here. In addition, I would point out that if approximately 95% of the mass in the universe is negative rather than positive, then perhaps its actually 100% -- a perfectly equally balanced universe of positive and negative mass, which seems rather elegant.
  5. Dec 6, 2018 #4


    User Avatar
    Science Advisor

    How is 20:0 less imbalanced than 19:1, let alone perfectly balanced?
  6. Dec 6, 2018 #5
    Because a generic property of inflation is that the universe began from a small quan-
    tum fluctuation. According to Vilenkin, “A small amount of energy was
    contained in that [initial] curvature, somewhat like the energy stored in a strung bow. This ostensible violation of energy conservation is allowed by the Heisenberg uncertainty principle for sufficiently small time intervals. The bubble then inflated exponentially and the universe grew by many orders of magnitude in a tiny fraction of a second.”

    If the there is the same amount of negative mass as positive mass then the sum total is zero and that makes the idea that the whole thing sprang into existence out of some vacuum potential more plausible to me
  7. Dec 6, 2018 #6


    User Avatar
    Science Advisor

    But the point is, the paper doesn't claim there to be the same amount of positive and negative mass. It claims that all the energy density currently attributed to dark energy + dark matter is negative mass density. So, it proposes an already unbalanced state with a 19:1 ration of negative:positive mass.
    A 100% negative mass density would have 0% positive mass in it.

    Also, if I'm reading this paper correctly, this model does away with inflation, going for cyclic cosmology instead.
  8. Dec 6, 2018 #7
    I'm surprised this hasn't yet generated more discussion. Seems like an elegant theory.
  9. Dec 6, 2018 #8
    This is the most interesting new hypothesis I have come across in a long while, the idea that a single hypothetical substance (field/particle) could explain the two phenomena that are currently explained by dark energy and dark matter separately. It is this that J.S. Farnes (https://www.researchgate.net/profile/Jamie_Farnes) explains in his paper which is on Arxiv at https://arxiv.org/pdf/1712.07962.pdf

    The first thing I thought was "how can a single substance explain both the 'missing mass' that is needed for anomalous galaxy rotation and accelerating expansion of the Universe?" The answer is given rather clearly in the paper by an extremely simple diagram whose most important feature is the fact that with a material which has negative mass, force and acceleration act in opposite directions (Newton would have been happy with this, since it is what F=ma says!). The result is that negative mass, which has positive energy according to Einstein's energy momentum equation (and which makes E=mc^2 a wrong simplification for negative mass!) repels itself, but is attracted to positive mass.Positive mass is repelled by negative mass, but since there is negative mass in all directions, this does not make galaxies fly apart. Farnes assures us that the calculations show it can explain the effect for which dark matter was invented and easily explains the accelerating expansion of the Universe.

    Farnes does not claim the hypothesis is definitely true: he identifies experimental tests that can test his hypothesis and discriminate between it and other more established ideas.
  10. Dec 7, 2018 #9
  11. Dec 7, 2018 #10


    User Avatar
    2017 Award

    Staff: Mentor

    Several threads about this subject have been merged.
    Because of the highly speculative nature of this paper, I want to remind all participants to stay as close as possible within a debate about the publication.
  12. Dec 7, 2018 #11


    Staff: Mentor

  13. Dec 7, 2018 #12


    Staff: Mentor

    One thing I am having trouble understanding in the paper: The author appears to be arguing that negative mass can have the same effect as a negative cosmological constant. But I don't see how he is arriving at that result. A negative cosmological constant is a negative energy density, but a positive pressure of equal magnitude, so the "source" term ##\rho + 3 p## is positive. A negative mass, as he is modeling it, appears to be a negative energy density with zero pressure, which makes ##\rho + 3 p## negative. So a negative mass should have the opposite effect to a negative cosmological constant.

    This apparent error seems to me to be related to what I find to be a glaring omission throughout the paper: the author only considers the first Friedmann equation and never considers the second (he writes the second down as equation 3 and then never mentions it again). But a proper understanding of the dynamics requires both equations.
  14. Dec 7, 2018 #13


    Staff: Mentor

    If you mean ##E^2 - p^2 = m^2##, that equation can't tell you about the sign of the mass or the energy or the momentum, since all of them are squared.

    The paper says that negative mass means negative energy density ##\rho## in the Friedmann equations, which are derived from the Einstein Field Equation. So according to the paper, negative mass has negative energy.
  15. Dec 7, 2018 #14
    Chris, based on the symmetry and the basic dynamics, the effect of the hypothetical negative mass on space-time has to be the opposite of that of positive mass. . For example, regions near a positive mass have to experience gravitational blue shift compared to regions near a negative mass.

    The question is really what is the stress energy tensor for a negative mass density. I believe it has to be -1 times the stress-energy tensor for a positive mass with similar distribution to have the correct properties.
  16. Dec 7, 2018 #15


    Staff: Mentor

    Just knowing the energy density can't tell you the stress-energy tensor. You have to make an assumption about it. The assumption the paper appears to be making is that "negative mass" means negative energy density ##\rho##, and zero pressure, similar to ordinary matter having positive energy density ##\rho## and zero pressure. Basically, this means that the negative mass the paper postulates, like ordinary matter, is cold.
  17. Dec 7, 2018 #16
    Yes, Peter. I had spotted my mistaken understanding about the sign of the energy when I couldn't edit it, and you are right that it is merely that the energy-momentum equation is consistent with either sign of energy rather than implying it is positive. A question is whether all positive energies breaks any conservation laws.
    My previous post was saying exactly the same as yours: some assumption has to be made about the stress energy tensor. However, that assumption is strongly guided by the basic dynamics you want to produce by definition of what a negative mass is, and assumed symmetries probably provide the rest.
    Last edited: Dec 7, 2018
  18. Dec 7, 2018 #17
    Great respect for Sabine Hossenfelder. I found her response to a poster named DreamChaser to be of particular interest, DreamChaser said that they liked the study because it was elegant in offering an explanation for both dark energy and dark matter (I will admit that I also found this appealing) and was therefore simpler solution. This was her reply:
    Sabine Hossenfelder said...


    It does not, please look at the paper. You need to introduce some weird stuff new stuff, then you need to introduce the creation tensor, then you need to assume you have no problem with vacuum stability, then you need to somehow assume that you get around the issue with the spin-2 field while still using GR, then you need to explain how come that a negative cosmological constant is actually in agreement with all the data, and even if you have done that you'd still have to bend over backward to demonstrate that the solution actually does fit the rotation curves which, frankly, I am rather skeptic about because I cannot see how you get the right scaling behavior (Tully-Fisher and all). Best,

  19. Dec 7, 2018 #18
  20. Dec 7, 2018 #19
    Sean Carroll's main point is why I am uncomfortable with negative energy and why I assumed (inconsistently with Franes, it seems) that the energies are all positive and it is only the gravitational interaction that is flipped. I am yet to be completely convinced that this is impossible, but that is not really for this discussion.
    [Doing a back of an envelope calculation, I find that the simple dynamics based on gravitational interactions as described in the paper conserve energy and momentum with either sign of energy associated with the rest masses. The weird feature is that the total energy of negative mass goes down as it goes faster, because the (m v2) / 2 term in a Newtonian approximation is negative). This is what makes it possible to conserve energy in the situation where a negative mass chases a positive mass. Another concern of mine is whether with such weird dynamics any system with both types of matter could be stable].
    Last edited: Dec 7, 2018
  21. Dec 7, 2018 #20
    Bandersnatch, yes I see what you mean about the positive/negative mass ratio...I had it wrong

    I have been thinking about some of the interesting implications/predictions of this theory:

    1) If this is dark matter, this explanation tells us why it is "dark" -- specifically, dark matter would be "dark" because these negative mass particles are mutually repulsive, they would never clump together into anything that would reflect light. Additionally, because most negative mass gets ejected outside of the galaxy into the halo, we wouldn't expect there to be much of it nearby to Earth.

    2) I found it interesting in section 3.4 where it is pointed out that the runaway motion that is described for positive-negative mass pairs could explain cosmic rays. This theory should be capable of predicting the frequency or density of cosmic rays as a function of one's location in space, which could potentially be used as an additional testable prediction.

    3) Because positive masses attract and clump together, I think this implies that any positive mass objects in free space would experience some amount of uniform pressure from all directions from negative mass particles that are colliding with it. It begs the question what these collisions would look like. Since the positive mass particles are clumped closely together, I would think that the EM force would hold them together strongly, and so negative mass particles wouldn't be able to break apart positive mass objects, since gravity is weaker than EM.

    4) It would make intergalactic space travel significantly more difficult. In addition to the already known difficulties of needing to escape the planet's gravitational field, and then needing to escape the gravitational attraction of the galaxy you are trying to leave, you would also need to fight your way though the repulsive force of this very large negative mass halo. If you did succeed in crossing the tipping point in getting far away from your home galaxy, once you got outside of the negative mass halo it would then provide a propulsive effect...but entering a new galaxy would again be very difficult.

    5) The author explains that this theory corresponds to an Anti-de Sitter space which undergoes a cycle of expansion and contraction. It seems not hard to visualize why this would be so...if we assume all positive mass fits within some finite extent, and negative mass is always created within this radius driving expansion, the negative mass would eventually be ejected outside of the positive mass universe and form a large scale halo of negative mass surrounding the entire universe in the same way that it is proposed to do for galaxies. Eventually, the ratio of negative mass "inside" the universe vs "outside" would change, until it reaches a tipping point where there is so much negative mass outside the universe that it begins to compress and slow down the inflation, and then crunches everything back down.

    6) If this theory is correct I think it also tells us what we could expect to happen if someone tried to fly "to the edge of the universe"...basically, it would become increasingly difficult to travel beyond the edge of positive mass, because you'd be fighting against the repulsion of the negative mass halo, so you'd basically just fly until the repulsive mass effect canceled out whatever you were using for thrust.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted